1) Field of the Invention
The present invention relates to an image processing apparatus, particularly, to a filtering processor that switches between two filtering positions, the positions being before compression and after expansion of image data.
2) Description of the Related Art
An image processing apparatus, such as a scanner and a copying machine, reads an image data, compresses the image data, stores the image data compressed in an image memory, read the image data from the image memory, and expands the image data read. The image data expanded is further recorded in a memory or sent to an external device such as a computer.
Two compressions of image data, reversible and irreversible compressions, are known. The irreversible compression differs from the reversible compression in that the image data before and after compression and expansion are not the identical.
An image processing apparatus, which processes digital data, has a filtering function (e.g., an edge highlighting) as one of important functions.
A multifunction products (hereinafter, “MFP”) system, which stores multi-level images and has multiple functions such as scanning, facsimile, and printing functions, generally performs either pre-filtering or post-filtering.
In the pre-filtering, as is shown in FIG. 9, an MFP system 100 reads image data from a reader 101 and filters the image data by passing through a filtering processor 102. The image data filtered is encoded by a data compressor (encoder) 103 in each 8 bit and is then stored in an external storage 104. The MFP system 100 then reads the 8 bit data from the external storage 104 and expands the 8 bit data by a data expandor (decoder) 105. The 8 bit data expanded is subjected to gradation conversion (dithered) by a gradation converter 106 and then printed out by a writer 107.
On the other hand, the post-filtering is shown in FIG. 10. Same reference numerals are used for same components in FIG. 9. In the post-filtering, the image data read from the reader 101 and the 8 bit data compressed by the data compressor 103 are stored in the external storage 104. The MFP system 100 then reads the 8 bit data from the external storage 104 and expands the 8 bit data by the expandor 105. The 8 bit data expanded is filtered by the filtering processor 102. The 8 bit data filtered is subjected to gradation conversion (dithered) by the gradation converter 106 and then printed out by the writer 107. 10.
Further, when the compression and expansion by the data compressor 103 and the data expandor 105 conform to the reversible compression, the same print output is obtained in both the pre-filtering and the post filtering. However, when the compression and expansion conforms to the irreversible compression, the print output is different in the pre-filtering and the post filtering, resulting in different image quality. This is because the frequency component of the image data is partially lost so as to reduce the size of the image data.
Thus, in a MFP system that employs the irreversible compression, the characteristics of output data are affected considerably depending on whether the filtering of the image data is the pre-filtering or the post-filtering. In other words, the filtering position is an important factor.
The irreversible compression, in many cases, makes a loss of the frequency component to the image data, thereby resulting in a phenomenon like a tendency to decrease in the edge sharpness of a character.
To correct the decrease in the edge sharpness, the filtering processor 102 as shown in FIG. 11 performs the strong edge highlighting on image data read from the reader 101, in case of the loss of the frequency component due to the irreversible compression. The image data filtered is compressed by the data compressor 103 and stored in the external storage 104. The image data stored in the external storage 104 is expanded by the data expandor 105 and then is processed in a printer γ converter 108. After that, the image data is processed in the gradation converter 106 and then printed out by the writer 107. As a result, sharpness of characters is compensated. In the other words, even if edge data of a character is partially lost due to the irreversible compression, the improvement in sharpness of a character is not restricted and the image quality can be improved.
Moreover, when the image process is switched (hereinafter “adaptive γ conversion”) according to judgment whether on edge area or on no edge area (inside) of a character (e.g., a printer γ conversion table for adjusting the output density), to cut the cost down, the judgment is performed by using a first in first out (hereinafter, “FIFO”) buffer for the filtering.
For example, as shown in FIG. 12, the image data read from the reader 101 is compressed by the data compressor 103 and then stored in the external storage 104. The image data stored in the external storage 104 is expanded by the data expandor 105 and filtered by the filtering processor 102. For the image data filtered, the judgment of whether on edge area or on no edge area (inside) is performed by using a FIFO buffer of the filtering processor 102. According to the judgment, the image data is subjected to adaptive γ conversion by the adaptive γ converter 109. Then, the image data is subjected to gradation conversion by the gradation converter 106 and print output is obtained in the writer 107. The γ conversion table depends on whether the image data is in the character area or in the picture area. Therefore, it is possible to obtain image density that is suitable to the corresponding area, so that the quality of mixture images of characters and pictures are improved.
However, since the printer γ conversion is a function to adjust the density of printing, in an image processing apparatus that stores 8 bit data like the MFP system 100, the printer γ conversion must be performed in the subsequent stage.
Moreover, the post-filtering requires that both the filtering and the adaptive γ conversion are performed in the subsequent stage of the compression and expansion.
Therefore, the conventional image processing apparatuses as described above, to improve the image quality, require switching between the pre-filtering and the post-filtering according to the image data. However, to switch between the pre-filtering and the post filtering, there is a need to provide two filtering processors 102a and 102b respectively at the previous and subsequent stages of compression and expansion, as shown in FIG. 13. Moreover, each of the processors 102a and 102b must include the FIFO buffer. Such an image processing apparatus is high cost.